High frequency electron discharge device



Jan. 26, 1960 BROCK HIGH FREQUENCY ELECTRON DISCHARGE DEVICE Filed Feb.24, 1953 2 Sheets-Sheet 1 WSW MMV,

Jan. 26, 1960 BRUCK HIGH FREQUENCY ELEcIRoN DISCHARGE DEVICE 2Sheets-Sheet 2 Filed Feb. 24, 1953 IN VEN TOR.

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States latent 5 Ztl-22,919 Y Patented Jan. 26, 1960 i ice acts as atravelling wave tube section, having a signal 2,922,919 wave excitedtherein whose propagating velocity along the section is equal to theelectron beam velocity pass- C ing therethrugh Lothar Brck, Ulm(Danube), Germany, assignor to Telefunken Gesellschaft fuer drahtloseTelegraphie G.m.b.H., Hannover, Germany Application February 24, 1953,Serial No. 338,499 Claims priority, application Germany February 25,1952 13 Claims. (Cl. S15-3.6)

This invention relates to an electron discharge device for generatingultra-short frequencies, and in particular to a discharge device whichcomprises an electron wave tube section and a travelling wave tubesection combined in one envelope.

The use of travelling wave tube for generating high power output levelsis well known. It is also well known that certain intrinsic difcultiesexist in the design of such a tube, these difiiculties involving (l) Themanufacturing of a uniform helix winding;

(2) Maintaining the helix in axial alignment;

(3) Reducing the interception of the electron beam by the helix to aminimum, particularly at very high frequencies.

The electron wave tube is equally well known and involves the use of twoelectron beams having different velocities which have impressed thereon,by means of suitable electromagnetic circuits, high frequency signals tobe amplified. While it is true that high voltage ampliiication may beobtained with the electron wave tube, the power output from such tubesis limited while the high voltage amplification is predominant.

It is therefore an object of this invention to provide a high frequencyelectron discharge device which utilizes the advantageous features ofthe electron and the travelling wave tubes.

It is another object of this invention to provide a high frequencydischarge device which may be easily manufactured.

It is yet another object of this invention to eliminate thedisadvantages associated with electron and travelling Wave tubes and toutilize the best features thereof.

With the above objects in view the present invention consists mainly ofan electron discharge device for generating ultra-short frequencies,comprising in combination, a vacuum tight envelope, a first and a secondelectron beam emitting means located in the envelope, emitting twoelectron beams having different velocities, collector means located inthe envelope, the collector being arranged opposite to and spaced fromthe rst and second electron emitting means to define a common path forthe electron beams, input delay line means arranged in the region of thecommon path of the electron beams after the same has been emitted by theelectron emitting means, the input delay means adapted to be coupled tosignal waves to be amplified so as to velocity modulate the electronbeams, output delay line means having a pre-determined dimension, theoutput delay line means being arranged in the region of the path of theelectron beams subsequent to and spaced from the input delay line means,so as to leave between the input and output delay line means a fieldfree region wherein the velocity modulated electron beams aretransformed to density modulated electron beams, a voltage sourceconnected to the output delay line means, the voltage source applyingsuch a potential to the output delay line so that the output delay linedue to its predetermined dirnension and the potential applied theretoThe novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantage thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings, inwhich:

Fig. 1 illustrates schematically a high frequency discharge deviceconstituting one form of the invention;

Fig. 2 illustrates a curve showing the relative amplitude versuselectron volts in the output section of this high frequency dischargedevice; and

Fig. 3 illustrates a modification of the invention wherein two electronbeams are generated, one of these beams surrounding the other.

Referring to Figure 1, a high-frequency discharge tube 21 is shown, intowhich is introduced through its input connector 1, 1 a signal to beamplified. The. outer conductor 1 of the input coax connector is vacuumsealed to the glass envelope 11 of the discharge)I device, the centerconductor 1 of the input connector 'being connected through a vacuumseal in the envelope to the input end of the helix or delay transmissionline 2. At the opposite end 7 of the delay line 2 is con# nected amatched attenuator 9 in order to prevent reflection of the input signalin the event of a mismatch. An output connector 20, 20 is vacuum sealedat its outer conductor 20 to the glass envelope 11, its inner conductorbeing connected through a vacuum seal in the envelope to the output endof a second helix, or a delay transmission line 5. At the opposite end 8of the helix 5 is connected a matched attenuator 10 to preventreflection of amplified signals from the output toward the input end ofthe tube. Electron streams 3 and 4 are emitted by cathodes 19A and 19Brespectively. The filaments of cathodes 19A and 19B are connected tovoltage source 14 and 15. Between voltage source 14 and 15 is connecteda voltage source 16 in such manner that cathode 19B will be at a higherpotential than cathode 19A with respect to the common return point 16a.Under these conditions the electron stream emitted by cathode 19B willhave a higher velocity than the electron stream emitted by cathode 19A.Another voltage source 17 is connected at one of its terminals to thesource 16 vat point 16a, which is the mid-voltage point of source 16,the other terminals of voltage source 17 being connected to theaccelerating anode 19. The input side of delay transmission line 2, isconnected to the accelerating anode, 19, and voltage source 17, via aradio frequency choke 12. This choke is used to prevent anyhigh-frequency signals from reaching the D.C. voltage source. Voltagesource 18 is connected in series with voltage source 17 at one terminalthereof, and at the other terminal thereof to a collector 6.The-collector 6 in turn is connected to the inner conductorv 20 of theoutput connector via radio frequency choke 13, thereby placing the helixor delay line 5, at the same potential as the collector 6. The voltageoutput of source 18 is approximately 2000 volts positive with respect tocathodes 19a and 19b. It is to be understood that this voltage is ofcourse merely illustrative, and is not to be construed as a limitationof the invention.

The operation `of the discharge device illustrated in Figure 1 is asfollows:

input signal will be impressed on each of the electron streams 3, 4, byAmeans of the delay line 2, thereby velocity modulating the same. Theelectron streams having ditferent average velocities upon becomingdensity modulated interact so as to produce an amplified Wave. Thefdelay line 2,- as noted, is used to impress a signal ontherelectronstreams, the amplification, however, taking place in the electron flowitself. The important factor in the interaction is the electric field,which stores energy Vand, acts on the electrons, and the electronsthemselves.

The charge of the electrons produces the electricv field; the mass ofthe electrons, and their kinetic energy, serve as do inductances andmagnetic stored energy in electromagnetic propagation. In electronl wavetubes it is customary to-use relatively low voltages'for acceleratingthe electrons in an electron stream which produces, as a result, lowbeam current, thereby yielding only a relatively low poweroutput. Theweak beam current is acted upon by the impressed high frequency signalwhich interacts therewith, thereby creating strong modulation of thebeam. In the ield free region, between delay lines 2 and 5, theelectrons become density modulated by virtue of the faster electronsVovertaking the slower `electrons and forming concentrated bunches. Theelectron streams 3, 4 emitted by cathodes 19A and 19B respectively,after passing through the iield free region come under the influence ofthe high potential connected to helix 5 and collector 6. As a result ofthe Yhigh potential at helix 5 and-collector 6, the variation-invelocity of the electrons in electron streams 3, 4 become practicallyinsignicant, so that the section of the discharge tube including helix5, attenuation layer 110, and collector 6 acts as a travelling waveampliier. To insure the operation ofthis section ofthe tube asatravelling wave tube, the length, the dimensions (pitch and diameter)and the propagation conditions are fixed so that the waves excited inthis section of the tube by electron streams 3, 4 will propagate at thesame velocity as the electron streams 3, 4 passing therethrough.

Figure 2 illustrates in graphic form why the output section of thehigh-frequency discharge device 21', operates as a travelling wave tube.In this ligure, the relative amplitude in watts is plotted againstelectron volts. The electron velocity difference of 50 volts is Vseen tobecome insignificant under the iniiuence of the 2000 volts connected tothe delay line 5 and collector 6.

As stated the attenuation layers 9 and 10 prevent high frequency signalsfrom being reflected beyond the ends 7, 8 of delay lines 2,5,respectively, in the event of any mismatches in the tube. In order toreduce the effect of the attenuation layers on the electron streams 3,4, it is also possible to gradually increase the diameter and pitch ofthe helices in the vicinity-of ends 7, 8 and use a ceramic wedge coatedwith an aquadag suspension to eliminate high-frequency signalreflections. The remaining part of the helix ,may be wound uniformly. Itis of course to be understood that the delay line is not limited to theuse of a conductor wound into a helical form but includes all highfrequency structures capable of reducing the phase velocity of highfrequency waves propagated thereby. Examples of some of these well knowndelay structures, are for instance, periodically loaded lines in`volving capacitive or inductiveV loading or made of circular links.

Figure 3 illustrates a modification of described in relation to Figures1 and 2.

In thisy ligure, unlike Fig. 1, where the cathodes were disposed side byside resulting in two electron streams moving alongside each other,cathode 22 is centrally located in relation to cathode 21. Cathode 21 isdesigned to emit a hollow beam (not shown) which encircles the beamemitted by cathode 22 (not shown). These electronbeams are collected atcollector 32. Delaylines 25 and 27 are supported by means of rods 23and24 respectively. These rods are provided at one end thereof lwithattenuation layers '26, 28, respectively, which serve to the inventionprevent micro-wave signal reflections. The voltage sources 14, 15, 16,17 and 18 are connected in the same manner as they Were in theembodiment described in relation to Figure 1. The same potentials forexplanatory purposes are applied in this embodiment to the acceleratinganode 31 and to the collector as in the embodiment illustrated in Figurel. An input connectorV 29, 29 is illustrated, the outer conductor ofwhich, is joined to the glass envelope 33, the inner conductor of whichis connected through a vacuum seal in the envelope, to'th'e input end ofthe delay line 25. An output connector 30, 30 is likewise connected tothe glass envelope 33 at its outer conductor, its inner conductor beingconnected similarly through a vacuum Seal in the envelope to the outputend of delay line 27.

The potential and Vthe size of the output delay line are again chosen,according to the invention, so that the delay line section 2.7 operatesas a travelling wave tube section. Y The operation of the embodimentillustrated in Figure 3 is similar to the operation described inrelation to Figure 1. The advantage of the arrangement illus'- trated inFigure 3 arises from the closer contact between the two electron streams(not shown), emitted by cathodes 21, 22thereby resulting in greaterinteraction.

I claim:

l. An' electron discharge device for generating ultrashort frequencies,comprising in combination, a vacuum tight envelope; a iirstrand a secondelectron beam emitting means located in said envelope, emitting twoelectron beams having different velocities; collector means located insaid envelope, said collector being arranged opposite to and spaced fromsaid first and second electron emitting means to definea common path forsaid electronV beams; input delay linemeans arranged in the region ofsaid common path of said'electron beams after the same has been emittedby said electron emitting means, said input delay means adapted to becoupled to signal waves to be amplied so as to velocity modulate saidelectron beams; output delay line means arranged in the region of saidpath of said electron beams subsequent to and spaced from said inputdelay line means, sogas to leave between said input and output delayline means a ield free region acting as a traveling wave tube sectioncausing predominantly voltage amplification wherein the velocitymodulated electron beams are transformed to density modulated electronbeams; a voltage source connectedV to said output delay line means forapplying a direct potential thereto which is ,substantially higher thanthe potential of' said input delay line in such mannerthat said outputYdelay line means acts as a traveling iield tube section causingpredominantly power amplification and a signal wave excited therein ispropagated therealong at a velocity approximately equal to the'electronbeam velocity passing therethrough.

2. An electron discharge device for generating ultrashort frequencies,comprising, in combination, a vacuum tight envelope; a lirst and asecond electron beam emitting means located in said envelope, emittingtwo electron beams having differentrvelocities; collector means locatedin said envelope, said collector being arranged opposite to and spacedfrom said first and second electron emitting means to define a commonpath for said electron beams; an input helix arranged in the region ofsaid common path of said electron beams after the same has been emittedby said electron emitting means, said input helix adapted to be coupledto signal waves to be amplified so as to velocity modulate said electronbeams and to act as a traveling wave tube section causing predominantlyvoltage amplification; an output helix having a predetermined dimension,said output helix being arranged in the region of said path of saidelectron beams subsequent to and spaced from said input helix, so as toleave between said input and output helices a field free regionWhereinthe velocityV modulated electron beams are transformed to densitymodulated electron beams and amplify .ne an'.

'said signal waves; a voltage source connected to said output helix forapplying va direct potential thereto which is substantially higher thanthat of said input helix in such manner that said output helix acts as atravelling field tube section causing predominantly power amplificationand a signal wave excited therein is propagated therealong at a velocitysubstantially equal to the electron beam velocity passing therethrough.

3. An electron discharge device for generating ultrashort frequencies,comprising, in combination, a Vacuum tight envelope; a first and asecond electron beam emitting means located in said envelope, emittingtwo electron beams having different velocities; collector means locatedin said envelope, said collecor being arranged opposite to and spacedfrom said first and second electron emitting means to define a commonpath for said electron beam; input delay line means arranged in theregion of said common path of said electron beams after the same hasbeen emitted by said electron emitting means, said input delay meansadapted to be coupled to signal waves to be amplified so as to velocitymodulate said electron beams and to act as a traveling wave tube sectioncausing predominantly voltage amplication; output delay line meanshaving a predetermined dimension, said output delay line means beingarranged in the region of said path of said electron beams subsequent toand spaced from said input delay line means, so as to leave between theadjacent ends of said input and output delay line means a eld freeregion wherein the velocity modulated electron beams are transformed todensity modulated electron beams; attenuating means connected at theadjacent ends of said input and output-delay line means to preventintercoupling of said input and output delay lines; a voltage sourceconnected to said output delay line means for applying a directpotential thereto which is substantially higher than the potential ofsaid input delay line means in such manner that said output delay linedue to its predetermined dimension and the potential applied theretoacts as a travelling Wave tube section causing predominantly poweramplification, and a signal field excited therein is propagatedtherealong at a Velocity substantially equal to the electron beam andamplify said signal waves velocity passing therethrough.

4. An electron discharge device for generating ultrashort frequencies,comprising, in combination, a vacuum tight envelope, a first electronemitting means; a second electron emitting means arranged in coaxialrelation to said first electron emitting means, said first and secondelectron emitting means located in said envelope and emitting twoelectron beams having different velocities, one of said electron beamspropagating within the other; collector means located in said envelope,said collector being arranged opposite to and spaced from said first andsecond electron emitting means to define a common path for said electronbeams; input delay line means arranged in the region of said common pathof said electron beams after the same has been emitted by said electronemitting means, said input delay means adapted to be coupled to signalwaves to be amplified so as to velocity modulate said electron beams andto act as a traveling wave tube section causing predominantly voltageamplification; output delay line means arranged in the region of saidpath of said electron beams subsequent to and spaced from said inputdelay line means, so as to leave between said input and output delayline means a field free region wherein the velocity modulated electronbeams are transformed to density modulated electron beams and amplifysaid signal Waves; a voltage source connected to said output delay linemeans for applying a direct potential thereto which is substantiallyhigher than the potential of said input delay line in such manner thatsaid output delay line acts as a travelling field tube section causingpredominantly power amplification and a signal wave excited thereintravels therealong at a velocity approximately equal to the electronbeam velocity passing therethrough, A

5. vAn electron discharge device comprising, in coibination, anelongated closure defining a path of travel for a stream of chargedparticles; electrode means located at one end yof said closure fordirecting a pair of closely coupled streams of charged particleslengthwise of said path at different velocities; velocity modulationmeans located near said one end of said elongated closer acting atraveling Wave tube section causing predominantly voltage amplificationfor velocity modulating said pair of streams of charged particles with asignal it `is desired to amplify, whereby said modulated pair of streamsof charged particles interact with one another to produce amplification;travelling field means causing predominantly power amplification spacedfrom said velocity modulation'means and located near the other end ofsaid elongated closure, said travelling field means having asubstantially higher potential than said velocity modulation means foraccellerating substantially said pair of streams of charged particlesand for deriving from said accelerated streams of charged particles aslow electromagnetic wave having a velocity in the direction of movementof said streams substantially equal to that of said streams, wherebysaid streams and slow electromagnetic wave interact to produceamplification; and means coupled to said traveling field means forderiving therefrom an amplified signal.

6. An electron discharge device comprising, in combination, an elongatedclosure defining a path of travel for a stream of charged particles;electrode means located at one end of said closure for directing a pairof closely coupled streams of electrons lengthwise of said path atdifferent velocities; velocity modulation means located near said oneend of said elongated closure acting a traveling wave tube sectioncausing predominantly voltage amplification for velocity modulating saidpair of streams of electrons with a signal it is desired to amplify,whereby said modulated pair of streams of electrons interact with oneanother to produce amplification; traveling field means causingpredominantly power amplification spaced from said velocity modulationmeans and located near the other end of said elongated closure, saidtravelling field means having a substantially higher potential than saidvelocity modulation means for accelerating substantially said pair ofstreams of electrons and for deriving from said accelerated streams ofelectrons a slow electromagnetic Wave having a velocity in the directionof movement of said streams substantially equal to that of said streams,-wlhereby said streams and slow electromagnetic wave interact to produceamplification; and means coupled to said traveling lfield means forderiving therefrom an amplified signal.

7. An electron discharge device comprising, in cornbination, anelongated closure defining a path of travel for a stream of chargedparticles; electrode means located at one end of said closure fordirecting a pair of closely coupled streams of charged particleslengthwise of said path at different velocities; velocity modulationmeans including delay line means located near said one end of saidelongated closure acting a traveling wave tube section causingpredominantly voltage amplification for velocity modulating said pair ofstreams of charged particles with a signal it is desired to amplify,whereby said modulated pair of streams of charged particles interactwith one another to produce amplification; traveling field means causingpredominantly power amplification including delay line means spaced fromsaid velocity modulation means and located near the other end of saidelongated closure, said travelling field means having substantiallyhigher potential than said velocity modulation means for acceleratingsubstantially said pair of streams of charged particles and for derivingfrom said accelerated streams of charged particles a slowelectromagnetic wave having a velocity in the direction of- `magneticwave interact to produce amplification;fand

Ameans coupled to said traveling field means for deriving therefrom anamplified signal.

8. An electron discharge device comprising, in combination, an elongatedclosure defining a path of travel for a stream of charged particles;electrode means located at one end of said closure for directing a pairof closely coupled streams of charged lparticles lengthwise of said pathat different velocities; velocity modulation means including a helix ofwire concentrically arranged with respect to the path traveled by saidstreams of charged particles and located near said one end of saidelongated closure acting a traveling wave tube section causingpredominantly voltage amplification for velocity modulating said pair ofstreams of charged particles with a signal it is desired to amplify,whereby said modulated pair of streams of charged particles interactwith one another to produce amplification; traveling field means causingpredominantly power amplification including a helix of wireconcentrically arranged with respect to the path traveled by saidstreams of charged particles, spaced from said velocity modulation meansand located near the other end of said elongated closure, said helix ofwire of said travelling field means being at a substantially higherpotential than said helix of wire of said velocity modulation means foraccelerating substantially said pair of streams of charged particles andfor deriving from said accelerated streams of charged particles a slowelectromagnetic wave having a velocity in the direction of movement ofsaid streams substantially equal to that of said streams, whereby saidstreams and slow electromagnetic wave interact to produce amplification;and means coupled to said traveling field means for deriving therefroman amplified signal.

9. An electron discharge device comprising, in combination, an elongatedclosure defining a path of travel for a stream of charged particles;electrode means located at one end of said closure for directing a pairof closely coupled streams of charged particles lengthwise of said pathat different velocities; velocity modulation means including delay linemeans located near said one end of said elongated closure acting atraveling Wave tube section causing predominantly voltage amplificationfor velocity modulating said pair of streams of charged particles with asignal it is desired to amplify, whereby said modulated pair of streamslof charged particles interact with one another to produceamplification; means for maintaining said delay line means at a givenpotential; traveling field means causing predominantly poweramplification including delayl line means spaced from said velocitymodulation means and located near the other end of said elongatedclosure; means for maintaining said last-named delay line means at apotential substantially more positive than that of said first-nameddelayV line means for accelerating substantially said pair of streams ofcharged particles and `for deriving from said accelerated streams vofcharged rparticles a slow electromagnetic Wave having a velocity in thedirection of movement of said streamssubstantially equal to that of saidstreams, whereby said streams and slow electromagnetic wave interact toproduce amplification; and means coupled to said traveling field meansfor deriving therefrom an amplified signal.

l0. An electron discharge device comprising, in cornbination, anelongated closure defining a path of travel for a stream of chargedparticles; electrode means located at one end of said closure fordirecting a pair of closely coupled streams of electr-ons lengthwise ofsaid path at different-velocities; velocity modulation means including ahelix ofV wire concentrically arranged with respect to` the pathtraveled by said streams ofrcharged particles and located 'near said one"end of said elongated closure acting a traveling wave tube sectioncausing pre- Cil dominantly voltage amplification for velocitymodulating said pair of streams of electronsY with a signalY it isdesired to amplify, whereby said modulated pair 'of streams of electronsinteract with one another to produce amplification; traveling fieldmeans causing predominantly power amplification including a helix ofwire concentrically arranged with respect to the path traveled by saidstreams of charged particles, spaced from said velocity modulation meansand located near the other end of said elongated closure, said helix ofwire of said travelling field means being at a substantially higherpotential than said helix of wire of said velocity modulation means foraccelerating substantially said pair of streams of electrons and forderiving from said accelerated streams of electrons a slowelectromagnetic wave having a velocity in the direction of movement ofsaid streams substantially equal to that of said streams, whereby saidstreams and slow electromagnetic wave interact to produce amplification;and means coupledV to said traveling field means for deriving therefroman amplified signal. Y

ll. An electron discharge device comprising, in combination, anelongated closure defining a path of travel for a stream of chargedparticles; electrode means located at one end of said closure fordirecting a pair oifsclosely ycoupled streams of charged particleslengthwise of said path at different velocities; velocity modulationmeans located near said one end of said elongated closure acting atraveling wave tube section causing predominantly voltage amplificationfor velocity modulating said pair of streams of charged particles with asignal it is desired to amplify, whereby said modulated pair of streamsof charged particles interact with one another to produce amplification;traveling field means causing predominantly power amplification spacedfromrsaid velocity modulation means and located near the other end ofsaid elongated closure, said travelling field means having asubstantially higher potential than said velocity modulation means foraccelerating substantially said pair of streams of charged particles andfor deriving from said accelerated streams of charged particles a slowelectromagnetic wave having a velocity in the direction of movement ofsaid streams substantially equal to that of said streams, whereby saidstreams and slow electromagnetic wave interact torproduce amplification;attenuation means operatively associated with s'aid `velocity modulationmeans and said traveling field means Vfor preventing intercoupling ofsaid two means; and means coupled to said traveling field means forderiving therefrom an amplified signal.

l2. An electron discharge device comprising, in combination, anelongated closure defining a path of travel for a stream ofchargedrparticles; electrode means located at one end of said closurefor directing a pair of closely coupled streams of charged particlesVlengthwiseof said path at different velocities; velocity modulationmeans including a helix of wire concentrically arranged with respect tothe path traveled by said streams of charged particles and located nearsaid one ,end of said elongated closure acting a traveling wave tubesection causing predominantly voltage amplification for velocitymodulating said pair of streams of charged particles with a signal it isdesired to amplify, whereby said modulated pair of streams of chargedparticles interact with one another to produce amplification; travelingfield means causing predominantly powerkamplification including a helixof wire concentrically arranged with respect' to the path traveled bysaid streams of charged particles, spaced from said velocity modulationmeans and located near the other end of said elongated closure, saidhelix of wire of said travelling field means being at a substantiallyhigher potential than said helix of wire of said velocity modulationmeans vfor accelerating substantially said pair ot' streams of chargedparticles and for deriving from said accelerated streams of chargedparticles a slow electromagnetic wave having a velocity in the directionof movement of said streams substantially equal to that of said streams,whereby said streams and slow electromagnetic wave interact to produce`amplification; attenuation means located at the ends adjacent oneanother of said two helices of wire for preventing intercoupling of saidvelocity modulation means and said traveling field means; and meanscoupled to said traveling iield means for deriving therefrom an ampliedsignal.

13. An electron discharge device comprising, nation, an elongatedclosure deiining a path of a stream of charged particles; electrodemeans one end of said closure for directing a pair coupled streams ofelectrons, one concentric other, lengthwise of said path at different incombitravel for located at of closely ywith the velocities;

velocity modulation means located near said one end of said elongatedclosure acting a traveling wave tube section causing predominantlyvoltage amplification for velocity modulating said pair of streams ofelectrons with a signal it is desired to amplify, whereby said modulatedpair of streams of electrons interact with one another to produceamplification; traveling ield means causing predominantly modulationpower amplification spaced from said velocity means and located near theother end of References Cited in the tile of this patent UNITED STATESPATENTS 2,238,770 Blumlein Apr. 15, 1941 2,406,370 Hansen et al. Aug.27, 1946 2,585,582 Pierce Feb. 12, 1952 2,652,512 Hollenberg Sept. 15,1953 2,652,513 Hollenberg Sept. 15, 1953 2,653,270 Kompfner Sept. 22,1953 2,694,159' Pierce Nov. 9, 1954 2,801,362 Hebenstreit et al July 30,1957 2,802,136 Lindenblad Aug. 6, 1957

